Phase transitions in ferroelectric thin films

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Abstract
We discuss theory and experiment for phase transitions in ferroelectric films, 70–400 nm thick. It is found that the Curie temperature Tc can be shifted up or down by temperatures as great as 70 K. In addition, transitions that are strongly first-order in bulk may become continuous in sufficiently thin films. Finally, the shape of the spontaneous polarization curve, plotted versus temperature, can become complicated, dropping to zero at an elevated Tc in two discrete jumps, with the higher temperature discontinuity arising from a residual ferroelectricity in a thick surface layer. These points are illustrated experimentally with data on PZT (PhZr1−x Ti x O3) and KNO3.